[wyvernfist]

Take a real bayonet, cut off all but 1/2 inch of the blade, then fit a piece of molded foam/plastic/rubber over that as a replacement blade?

 

[wyvernfist]

And after all that 1/2 inch would still count as a blade. Even in all the shortest shortness of it.

Sand off the edge of this 1/2 inch nub and call it an "external tang" :